It is pointed out in U.S. Pat. No. 3,661,859 that poly(parabanic acids) (PPA) are made by hydrolyzing poly(iminoimidazolidinediones) which are formed by the reaction of hydrogen cyanide with diisocyanates or by the reaction of dicyanoformamides with diisocyanates or by the polymerization of cyanoformamidyl isocyanates as fully set forth therein.
The hydrolysis reaction can be carried out by contacting a solution of a poly(iminoimidazolidinedione) with an aqueous solution of a Bronsted acid such as hydrochloric, hydrobromic, sulfuric, nitric, and the like, or with anhydrous hydrogen chloride or hydrogen bromide such that when the polymer is mixed with water, hydrolysis of the imino groups will occur to convert the polymer to the desired poly(parabanic acid) which is characterized by 1,3-imidazolidine-2,4,5-trione-1,3-diyl rings.
The solution of polyiminoimidazolidinedione may be the polymerization solution in which it was prepared or it may be one which was prepared by dissolving the isolated polymer in a preferable solvent.
When an aqueous solution of acid is added to a solution of the heterocyclic polymer characterized by imino-1,3-imidazolidinedione rings, the extent of hydrolysis may be controlled by the quantity of acid used. Complete hydrolysis requires a mole of hydrogen ion per mole of imino group to be hydrolyzed. The heterocyclic polymers before and after hydrolysis are both insoluble in water; therefore, the quantity of water used to dilute the acid must be limited so that precipitation of polymer does not occur.
To prevent precipitation of the polymer by localized high concentrations of water during the addition of the aqueous acid to the polymer solution, the aqueous acid may be first diluted with the same solvent in which the polymer is dissolved and this mixture then added to the polymer solution.
Hydrolysis occurs rapidly in the presence of an excess amount of acid and may be complete within a few minutes at room temperature. The general temperature conditions for hydrolysis are between 10.degree. and 120.degree. C., preferably between 30.degree. to 60.degree. C. The hydrolysis conditions may be maintained for a few minutes or for several hours. (See U.S. Pat. No. 3,661,859.)
Thus, poly(iminoimidazolidinediones) are readily converted to poly(parabanic acids) by acid hydrolysis. ##STR1##